1. Field of the Invention
The invention relates to the measurement of pressure within a liquid containing vessel. More particularly, the invention relates to a method for measuring pressure within the cardiovascular system of a living test subject.
2. Prior Art
The measurement of pressure within a liquid containing vessel such as the cardiovascular system of a living being can be performed by any of a number of methods. Catheterization and direct measurement of pressure is one wholly accurate method. Systemic blood pressure may also be measured externally of the test subject as by attaching a blood pressure cuff about a portion of the subject's body and listening for Korotkoff sounds, measuring the Doppler shift of the artery wall with fluctuating pressure, measuring the derivative of the instantaneous pressure at the cuff, and the like. Other methods of making such blood pressure determinations are discussed in, for example, "The Direct and Indirect Measurement of Blood Pressure" by L. A. Geddes, Yearbook Medical Publishers, Inc., Chicago, Ill., 1970.
The direct catherization method of measuring blood pressure has the advantage of being useable any place within the cardiovascular system, to the extent that irreversible physical damage and shock is not caused by the catheterization technique. The various techniques which require the use of a cuff can generally be used only on parts of the body where the cuff can be readily attached, for example, an arm, a leg, a finger, or the like. And, this approach is only amenable to the systemic circulation, for example, it cannot be used to measure pulmonic arterial pressure. Further, with small infants such as premature babies, it may be impossible, at times, to properly attach such a cuff. Hence, these techniques, while finding wide usefulness, are not useful to provide certain desired data such as the differences in blood pressure in different portions of the cardiovascular system, and the measurement of blood pressures in small infants, although the latter can be accomplished, at times, with special cuffs.
An attempt has previously been made to utilize the fact that when ultrasonic signals are reflected from microbubbles of known size, the frequency of the reflected signal will be a function of the pressure in the liquid in which the bubbles exist. If such bubbles are in a cardiovascular system of a living being, the frequency of the reflected signals from the bubbles will vary over a heartbeat in response to the variation in pressure within the cardiovascular system. This technique has been found, however, to be difficult to practice, because the ultrasonic signals have set up standing waves with the microbubbles, at times, at a node, and because the frequency of the signals has needed to be swept, which in combination with the existence of standing wave nodes has led to the detection of false signals. Thus, while one has been able in the past to inject preformed microbubbles into the cardiovascular system, and to reflect ultrasonic signals from these bubbles as they pass a desired position in that system, for example, the main pulmonary artery, the ultrasonic reflection system has not provided a signal which as a practical matter can be utilized for accurate measurements of blood pressure.
It is also known, as set out in copending U.S. Pat. application Ser. No. 36,098, of E. Glenn Tickner and Ned S. Rasor, to measure instantaneous blood flow rate in a cardiovascular system using microbubbles of uniform size in the system and to enhance ultrasonic images by injection of such microbubbles.
It would be highly desirable to provide a method for measuring pressure within a liquid containing vessel, particularly within the cardiovascular system of a living being, which method could measure the pressure in the system at different points therein and could be readily utilized with infants as well as adults. It would particularly be advantageous if such a system would allow measurement of pressure within the heart itself, without the necessity of performing a relatively risky catheterization and insertion of a pressure detector into the heart from a vein or artery. It would also be useful if blood flow rate and/or ultrasonic image enhancement could be provided with particularly advantageous signal-to-noise ratios.